First there was Kain Colter, a brawny Northwestern quarterback who wanted to form a union. Then there was Ed O’Bannon, a former UCLA basketball star who did not like seeing others make money by featuring him in a video game.

They both dealt serious blows to the foundations of the NCAA, foundations that rest upon the idea of the athlete as an unpaid amateur.

The NCAA’s most formidable opponent of all may be the one coming down the pike: a stout, 60-year-old antitrust lawyer from Brooklyn named Jeffrey Kessler.

In March, Kessler filed a lawsuit against the NCAA and the major college athletics conferences that he says will take down the “cartel” that controls college sports, and do away altogether with rules against paying college athletes.

College-sports experts see Kessler’s case as the biggest threat of all, and, with change in the air, they say he has reason to feel confident. If the NCAA has shown an inclination to tiptoe toward significant change, Kessler’s case takes a bazooka to the entire model of college athletics.

“What we are seeking to do is remove the shackles so that schools can decide themselves what is the fair and appropriate way to take care of their players,” Kessler said.

Kessler sees an open market that would allow colleges to compete for the services of top athletes by offering enhanced scholarships, better medical care — or cash. Potentially, lots of cash.

“If you have a school like Texas that earns close to $200 million for their football or basketball programs, you might decide it is fair to give some of that to the players who are generating the money,” he said. (Texas athletics generated $165.7 million in 2012-13, the university says.)

Kessler believes that there would be plenty of money to go around, and that it is only fair for a market to decide how athletes are compensated for their performance.

In Kessler’s future world, a top high-school basketball prospect could potentially be recruited by colleges just as the Cleveland Cavaliers courted LeBron James. There could be agents, and money, and maybe even a salary cap down the road. The prospect could choose where to go based on the coach, the facilities, the scholarship — or who is offering the most money.

“The players won’t get one dollar more than the markets decide they are worth,” Kessler said.

Kessler’s case is working its way through the court system. The judge has held preliminary hearings, and if the suit makes it past the legal challenges ahead, a trial could take place as soon as fall 2015.

Meanwhile, the NCAA has been under siege in the courts and in the court of public opinion, and the organization has taken steps toward change. The five wealthiest conferences recently gained the power to do more for their athletes, and a federal judge ruled that the NCAA’s ban on payments violated antitrust laws.

“Freedom case”

Kessler is not a wide-eyed activist. He was the lawyer who successfully negotiated the free-agency systems in the NFL and the NBA, models now at the core of professional sports. Now, he is looking to do the same at the university level.

Kessler filed the suit in federal court in New Jersey on behalf of Martin Jenkins, a former Clemson football player.

Kessler calls it “the freedom case,” but everyone else calls it the Kessler case. (It’s worth noting that in the other high-profile NCAA lawsuit, it was the plaintiff, O’Bannon, who took top billing, not the lawyer.)

With his Brooklyn accent and his Upper East Side address, Kessler doesn’t look like a sports heavyweight. He completed his undergraduate studies and law degree at Columbia University, an academic powerhouse that nevertheless occupies “a distant world from what my case is about.”

He is a self-described hacking golfer who as a child relied on his enthusiasm to overachieve at street football and basketball in the small community of Seagate, near Coney Island.

He heads the antitrust and competition practice at Winston & Strawn, often traveling to Japan to represent clients there. But he has long been drawn to the plight of athletes, a group he believes to have been historically disadvantaged. As a leading lawyer in the sports genre, Kessler has gained fame and made millions of dollars a year.

The college sports world is watching Kessler. Gerry Gurney, a former athletics executive at Oklahoma who opposes the professionalization of sports, fears that if Kessler wins, college sports will only become further removed from meaningful education.

“You can assuredly expect college sports to change drastically,” Gurney said. “It would certainly end all vestiges of the amateurism model.”

That, Kessler argues, is precisely the point. He says that those pushing an academic agenda in big-time college sports are living in a “world that doesn’t exist.”

“What they’d like to do is turn back the clock and not have the schools running their own cable-television networks, not having the schools earning all this money, not having the head coaches making $6 million or more,” Kessler said. “They want to go back to a world where there weren’t two gigantic businesses being run in football and basketball by those schools who run it.”

Two titans

No one is taking Kessler lightly. The NCAA has retained Jeffrey Mishkin, a longtime courtroom adversary of Kessler’s who has represented NBA owners in every major case they have faced in the past 35 years.

Mishkin this year called Kessler’s lawsuit “a sweeping attack on the continued importance and vitality of the principle of amateurism in college athletics.”

“Courts have upheld that principle against antitrust attack for more than 30 years,” said Mishkin, a partner with Skadden, Arps, Slate, Meagher & Flom. He declined to comment for this article because of the pending litigation.

The potential courtroom bout between Kessler and Mishkin, two titans in the legal sports world, sets up a dramatic showdown that could set the path for the future of college sports.

“I’ve thought it was like Ali-Frazier watching them go at each other with oral arguments,” said Ron Klempner, who was recently the interim executive director of the NBA players association and has partnered with Kessler for 25 years.

Kessler’s past cases not only led to the free-agency systems in the NFL and the NBA, but they also ended the 2011 lockouts in each sport. He has earned fans among prominent athletes. He likes recounting a conversation with Patrick Ewing, who told him: “I could see how you are great in the court, but I don’t think you’d be very great on the court.”

Kessler has closely watched the NCAA take on other adversaries.

A federal court ruling in the O’Bannon case Aug. 8 opened the door to players sharing in the revenues from broadcasts and video games. The case, which is being appealed, could make way for universities to set up trust funds for athletes to collect on their likenesses in broadcasts and video games.

Earlier, Northwestern football players won the right to form a union. That decision, which is being appealed to the full National Labor Relations Board, notably recognizes the players as employees.

Meanwhile, the NCAA agreed to settle lawsuits over its handling of concussions, yet another sign that the NCAA is trying to answer its critics.

“As time goes on, and as other decisions come down, they only make us feel more confident about the ultimate outcome,” Kessler said.

Kessler feels as if he might be on to something big — and on the right side of history. During a recent dinner with Fred Berkon, a friend of 20 years, the discussion turned toward the NCAA and Kessler told Berkon: “One day, kids will say, ‘You mean college players didn’t get paid at some point?’ ”